This section is intended to introduce the reader to various aspects of art, which may be related to various aspects of the present disclosure that are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
In the United States, the TV spectrum currently comprises Advanced Television Systems Committee (ATSC) broadcast signals that co-exist with National Television Systems Committee (NTSC) broadcast signals. The ATSC broadcast signals are also referred to as digital TV (DTV) signals. NTSC transmission is planned to cease in 2009 and, at that time, the TV spectrum will comprise only ATSC broadcast signals. However, and as was the case when only NTSC broadcast signals existed, in any given region of the country, significant TV spectrum goes unused in order to prevent interference between broadcast channels.
Recently, Government agencies and business have suggested that different services could, in the future, share frequency bands such as the broadcast TV spectrum. Various standards bodies have proposed a new wireless radio service known as a wireless regional area network (WRAN) that would share the TV spectrum currently used by terrestrial television broadcasting. One such proposed WRAN system is intended to make use of unused broadcast channels in the TV spectrum, on a non-interfering basis. The primary objective of the proposed WRAN system is to address broadband access in rural and remote areas and low population density underserved markets and provide performance levels similar to those of broadband access technologies serving urban and suburban areas. In addition, the proposed WRAN system may also be able to be scaled in order to serve denser population areas where spectrum is available.
In order for the WRAN system and the currently present broadcast signals to share the spectrum, interference between the two systems must be mitigated. It has been proposed that one way to control interference would be to ensure that in some instances the two services have orthogonal polarization of their respective signal radiation patterns. In the United States, broadcast TV signals are generally transmitted using horizontal polarization. A WRAN system seeking to operate in or around a broadcast channel used by a local broadcast station could be required to transmit using vertical polarization to minimize interference.
In order for WRAN devices, such as base stations and home premises equipment, to accomplish transmission using a particular (e.g. vertical) radiation polarization pattern, the antenna used by the WRAN device may require accurate alignment and may further be required to maintain accurate alignment if a WRAN transmission is to occur. One such measure of alignment may be to determine the amount of cross polarization isolation between, for instance, the vertical or horizontal radiation patterns. For example, a cross polarization isolation figure given as 14 dB may be used as a measure of correct alignment for proper radiation polarization and may be acceptable to ensure minimal interference between services.
One possible method of achieving the high level of cross-polarization isolation necessary to assure the desired radiation polarization of the antenna used with the WRAN device would be to install the antenna used by the WRAN device using a skilled, professional installer. The antenna could include a reference member that could be adjusted with a level or plumb bob or some electrical measurement device in order to attain a particular radiation pattern orientation. The adjustment and alignment of the antenna would be performed prior to permitting transmission by the WRAN device. Periodic skilled or professional adjustment would then assure proper alignment of the radiating elements to produce the desired cross-polarization and therefore assure the desired radiation polarization.
However, professional installation and continued professional monitoring and maintenance of the antenna used with a WRAN device may prove unnecessarily expensive. Additionally any adjustment of the antenna after the initial adjustment, due to environmental or other conditions would likely require the installer to return. Furthermore, if the antenna becomes misadjusted and is not radiating in the proper polarization orientation, unnecessary and undesirable interference with broadcast signals may result. It is therefore desirable to have an antenna system that can determine whether an antenna used with a shared wireless service, such as the WRAN service, is properly aligned particularly with respect to preventing interference with a shared spectrum service, such as broadcast television.